[R-390] Tube Shields/Temperature Instrumentation

[Bob Camp]

Hi,

A few observations:

Thanks to a twenty year old decision to stock up on tubes by our 
favorite pack rat government agency the world is awash in tubes for the 
R-390, R390A and a bunch of other stuff. Most of us have enough tubes 
to re-tube each of our radios a couple of dozen times.

Tubes are easy to replace. I have actually done it my self under a 
variety of less than optimum conditions *and* without any test gear at 
all. Tube swapping is a *lot* easier than any of the other maintenance 
procedures on one of these radios.

There are a couple basic parts of a tube as far as heat is concerned. 
You have the glass bulb, the seal between the glass bulb and the pins, 
and all the stuff inside. Each of these parts is affected by heat in a 
different way. They are also affected by a tube shield in a different 
way.

The glass bulb will melt if it gets to hot. I have seen a lot of weird 
things happen, but melting glass on receiving tubes is not one of them. 
The amount of power required to get the glass that hot simply isn't 
available. The glass does not seem to be terribly forgiving when 
exposed to a drop onto a concrete floor ...

The glass to metal seals on the tubes at the tube pins are an issue in 
terms of tube life. The glass and the metal have different thermal 
expansion coefficients. As temperature changes the seal is stressed. If 
the seal cracks even just a little you get a leak. Air inside a vacuum 
tube is not a good idea. I have seen tubes fail due to gas inside the 
tube. I'm not sure that the gas came from a seal leak though. The tube 
pins are heat sunk by the tube socket. They do not seem to be affected 
by the tube shield at all.

The guts of the tube do all the work and they are what usually fail due 
to heat. The filament obviously gets nice and warm in order to make the 
tube work. Hopefully nothing we do cools off the filament to much ... 
The rest of the parts get nice and hot and then cool down. Eventually 
the heat cycles make some of the grids sag or wires break. When they do 
the tube doesn't work as well as it might.

All of the magic tube shields cool the glass bulb. That's fine, but the 
bulb isn't the problem in the first place. The guts of the tube are the 
problem. The guts cool by radiation rather than by conduction or 
convection. It all gets down to a wonderful concept called emisitivity. 
Black tube shields are a good idea for radiant cooling. The only trick 
is they have to be black at infrared.

Simply cooling the guts of the tubes does not cool the radio at all. 
the heat is still inside the radio. You haven't even moved it around 
much. The rest of the parts in the radio are still nice and hot.  They 
are what you need to worry about. They are what's going to fail. They 
are what will be a real pain to replace.

Back in the good old days tubes cost a lot and the rest of the stuff 
was cheap. That included the labor to find a dead resistor. Times 
change ... The cost of that carbon comp resistor is a bit high now that 
they are no longer manufactured.

Forget about the tube shields. Buy fans. Buy lots of fans. Un-stack the 
radios. Open up the back door on the rack cabinet. Heat kills the rest 
of the parts in the radios. A single fan will do far more for a lot 
less money than a couple of fancy tube shields ...

	Enjoy!

		Bob  Camp
		KB8TQ